1. Field of the Invention
The present invention relates to a micro-mirror device, which can be used as a scanning mirror to be incorporated, for example, in a light scanning type sensor for form recognition, a bar code reader, or a laser printer, and to a method for producing such a micro-mirror device.
2. Description of the Prior Art
FIG. 17 is a schematic plan view of the micro-mirror device in the prior art. FIG. 18 is a cross sectional view of the micro-mirror device in FIG. 18, along the line 18--.about.of FIG. 7. A mirror portion 101 is disposed on a surface of a mirror forming substrate 102. The mirror portion is formed by a thin aluminum layer or a thin gold layer. The mirror forming substrate 102 can rotate around its center axis. A torsion beam 103 extends along the center axis of the mirror forming substrate 102. The torsion beam 103 is supported by a pair of anchors 104, which are fixed to a base substrate 106. A pair of driving electrodes 105 are disposed on the base substrate 106.
There are gaps of distance g0 between the driving electrodes 105 and the mirror forming substrate 102. A voltage is applied to either of the driving electrodes 105, so that the mirror forming substrate 102 is driven to rotate by the electrostatic force. The mirror forming substrate 102, torsion beam 103 and the anchors 104 are made from, for example, single crystal silicon, poly-crystalline silicon, or nickel plating. The base substrate 106 is made from, for example, silicon or glass.
The function of the micro-mirror device in the prior art is explained below.
When a voltage is applied to either of the driving electrodes 105, an attracting force is generated between the mirror forming substrate 102 and the driving electrode 105, depending on the voltage and the electrostatic capacity between them. The mirror forming substrate 102 rotates around its center portion, until the mirror portion 101 inclines at an angle .theta.s (scanning angle). The mirror portion 101 can be driven to rotate and swing simultaneously, when a voltage, for example, a superposition of a biasing direct voltage Vdc and alternating voltages Vac, having phases difference of 180 degree to each other, as shown in FIG. 19, is applied to the driving electrodes 105. The scanning angle of the mirror portion 101 and the scanning angle of a light beam can be controlled, depending on the imposing voltages.
When the micro-mirror device in the prior art is used, the theoretical maximum scanning angle .theta.smax of the scanning angle .theta.s is given by the following mathematical expression (1): EQU sin (.theta.smax)=g0/L (1)
where L is a distance between the center and the side end of the mirror forming substrate 102, as shown in FIG. 18. For example, assuming that L is 1 mm and the required maximum scanning angle .theta.smax is 15 degrees, the necessary distance go of the gap is calculated to be 259 .mu.m, using this mathematical expression (1).
However, the mirror portion 101 in an actual micro-mirror device in the prior art can not be rotated up to the full span, i.e., the theoretical maximum scanning degree. This is because the relation between the distance g0 of the gap and the electrostatic force to rotate the mirror portion 101 is non-linear, when the electrostatic force is used. Specifically, because the magnitude of the electrostatic attractive force is proportional to the inverse square of the distance g0 of the gap, when the inclination angle of the mirror forming substrate 102 is large and the distance go of the gap between the mirror forming substrate 102 and one of the driving electrodes 105 is small, the electrostatic attractive force between the mirror forming substrate 102 and the driving electrode 105 becomes larger than the restoring force due to the torsion of the torsion beam 103 at a large torsion angle. As a result, the mirror forming substrate 102 is fixed to one of the driving electrodes 105 and does not move. This phenomenon is called "Pulled-in Phenomenon". For avoiding this phenomenon, the scanning angle .theta.s of a light beam of the mirror portion 101 is restricted to be within a stable region, which is, in general, about a half of the theoretical maximum scanning angle .theta.smax.
The micro-mirror device in the prior art has the drawback that it is difficult to increase the maximum scanning angle .theta.smax, due to the Pull-in Phenomenon.
The other drawback of the micro-mirror device in the prior art is that it is difficult to design a micro-mirror device, which can scan at a wide range of scanning angle of a light beam, a low driving voltage is used. Because, assuming that the characteristics of the torsional oscillation, for example, shear modulus or the Q-value of the oscillation are constant, the range of a stable scanning angle and the corresponding driving voltage are determined by the size of the mirror forming substrate 102 with the mirror portion, and the distance g0 of the gap between the mirror forming substrate 102 and the driving electrodes 105, which are disposed under the mirror forming substrate 102.